It's more fun riding a slow bike fast than riding a fast bike slow.
If you buy any 600 cc sport bike you will be forced to ride the bike slow. And that is the problem with new riders. They have all this power and nowhere to use it. So an open stretch of road comes into view and the throttle is twisted wide open. Engine screams to redline in a blink of an eye. Shift to next gear and there is no let up on the acceleration. The speed and acceleration is intoxicating. You shift into 5th gear and look at the speedometer. Reads 140 mph. On a road with a 45 mph speed limit. So you let off the throttle and slow down to a more legal speed. Luckily you weren't in an accident or tagged by the police.
That is the problem ever time you ride a 150 plus mph sport bike.
I applaud your decision to start with a 250R verses the other bikes under consideration. That said, you are still in the learning phase of riding.
Here are a couple of suggestions. First, before you buy another bike, sign up for an advanced riding class. See first link.
Second, check insurance rates on a new or used 600RR. Rates will be huge due to your age and lack of riding experience.
Third, do you ever see yourself riding anywhere on a motorcycle? Say more than an hour or two? If so, why would you consider an expensive machine that will limit your ability to travel. There is no better way to SEE the country than from the seat of a motorcycle. That will be very difficult to do from the seat of a dedicated road racer. Take a little time and look through the Adventure Rider site. Second and third link below.
As to your question, buy used. Cheaper to buy used. Then use the saved money to pay for the higher insurance rates. If any money is left over sign up for the advanced riding school.
A 600 cc sport bike will not make you a better rider. The bike will hinder the learning process.
Sorry to be blunt, but this is the truth.